The present invention relates to a method of adjusting the pre-arcing time-current characteristic for a fuse and a fuse structure therefor.
Conventionally, an electric circuit for an automobile, etc. employs a fuse 1 as shown in FIG. 6 for protecting electric wires, devices, etc. from an excess current. The fuse 1 is formed in a manner that a pair of terminals 3 are coupled through a fuse element 5, both the terminals and the fuse element thus coupled are mounted within an insulative heat-resistant resin housing 7, and the upper potion 9 of the housing 7 is closed by a transparent cover 11. Accordingly, it is possible to visually confirm through the transparent cover 11 whether or not the fuse element 5 received within the housing 7 is fused or melted.
The fuse element 5 has a fusible portion formed by low melting point metal such as lead or tin which melts or fuses due to heat generated by the fuse element 5 when a current more than the rated current of the fuse element flows therethrough. When the fusible portion melts, the circuit connected to the fuse is opened thereby to protect the electric wires and the devices. Conventionally, the pre-arcing time-current characteristic of the fusible portion has been adjusted by changing the size (that is, the resistance value) of the fusible portion thereby to change an amount of heat generated therefrom.
A fuse has in general a constant relative relation between the conduction current and the pre-arcing time. That is, the fusible portion of the fuse melts immediately when there flows a current not less than twice as large as the rated current of the fuse (that is, dead short-circuit current). In contrast, the fusible portion of the fuse element 5 repeatedly generates and discharges heat when there flows a short-circuit current not more than twice as large as the rated current of the fuse or an intermittent short-circuit current (that is, rare short-circuit current). Accordingly, in this case, the pre-arcing time tends to become longer. Under such a circumstance, since the electric wires forming the circuits are covered by the insulating coating, unlike the fusible portion, the electric wires can not discharge the heat therefrom when the short-circuit current flows intermittently therethrough. As a consequence, the temperature of the electric wires increases continuously due to the accumulated heat therein, and so smoke, etc. may be generated from the electric wires if worst comes to worst.
However, according to the conventional adjusting method for the pre-arcing time-current characteristic, the pre-arcing time-current characteristic has been adjusted only by changing the size of the fusible portion. Accordingly, in order to solve the aforesaid problem, even if the size of the fusible portion is changed so as to change only the pre-arcing time-current characteristic in the rare short-circuit area shown in FIG. 7 from a characteristic curve A to a characteristic curve B, the pre-arcing time-current characteristic in the dead short-circuit area changes also from the characteristic curve A to a characteristic curve C. As a consequence, there arises a problem that a desired pre-arcing time-current characteristic can not be obtained.